An inverter for converting a direct current flowing between two input lines into an alternating current flowing between two output lines performing as a current source, and comprising a series circuit made of a first high frequency switch, an inductance and a second high frequency switch, and connected between the two input lines is known from DE 196 42 522 C1. Here, one of the output lines is directly connected to one of the input lines. An unfolding circuit of the known inverter comprises a first low frequency switch leading from the first intermediate point to the other output line, and a second low frequency switch leading from the second intermediate point to the other output line. Further, a third low frequency switch is provided between the second intermediate point and the one output line directly connected to the one input line. By high frequency operating the two high frequency switches on both sides of the inductance, the half-waves of the desired alternating current are formed. Here, the high frequency switch located on that side of the inductance from which the first rectifier diode leads to the first intermediate point is closed for those half-waves of the alternating current output at negative voltage between the output lines. During these negative half-waves, the first low frequency switch of the unfolding circuit is closed. The two other low frequency switches of the unfolding circuit are open. During the half-waves of the alternating current output at positive voltage between the output lines, the two high frequency switches connected in series with the inductance are operated at a high frequency, and the second low frequency switch of the unfolding circuit connected between the first intermediate point and the other output line and the third low frequency switch of the unfolding circuit connected between the second intermediate point and the one output line connected to the one input line are closed. Buffer capacitances are provided both between the output lines and between the input lines of the known inverter. In a real application of this known inverter, the buffer capacitance between the output lines is supplemented with an inductance in one of the output lines to make up an LC-filter for reducing a current ripple of the alternating current generated by the known inverter, i.e. for reducing variations of the alternating current at a high frequency due to its formation by means of high frequency switches. This current ripple is an essential disadvantage of the known inverter. Additionally, it has no flexibility with regard to earthing a photovoltaic generator connected to its input lines. Particularly, earthing is only possible at that input line which is directly connected to the other output line.
Depending on the type of solar modules of the photovoltaic generator, it may, however, be suitable to operate the photovoltaic generator either completely at negative or completely at positive electrical potential with regard to earth. Under certain conditions, like for example in order to limit the voltage with regard to earth, it may also be suitable to connect a center point of the photovoltaic generator to earth. This flexibility is not given with the known inverter.
An inverter comprising a series circuit made of a first high frequency switch, an inductance and a second high frequency switch and connected between two input lines is known from DE 10 2007 028 078 B4. A first rectifier diode leads from one side of the inductance to one terminal of a voltage link, and a second rectifier diode of opposite blocking direction leads from the second side of the inductance to the other terminal of the voltage link. The voltage link comprises a link capacitance; and a half-bridge made of two switches with anti-parallel diodes is connected between its terminals. A center point of the half-bridge is connected to the L-conductor of a connected AC power grid via a grid choke. The N-conductor of the AC power grid is connected to a center point of the link capacitance. The switches of the half-bridge are operated at a high frequency to shape an alternating current driven by the link voltage of the voltage link. Here, a grid choke is essential to limit the current ripple of the alternating current fed into the AC power grid. The known inverter with the pronounced voltage link is a typical representative of a voltage source inverter. The inductance of its series circuit may be coupled to a further inductance which is connected to the voltage link via an additional diode and which also loads the voltage link. In this embodiment of the known inverter, one of its input lines may be freely selected for being connected to earth.
EP 2 244 367 A1 discloses a boost converter assembly connected between a photovoltaic generator and a voltage link at the input side of an inverter bridge. The boost converter assembly comprises a plurality of boost converter units each comprising a controllable switch and an input inductor. The plurality of boost converter units are connected in parallel and provide a DC link voltage at the voltage link that is higher than a DC voltage provided by the photovoltaic generator. The individual boost converter units are operated in an interleaving mode.
US 2009/0251937 A1 discloses a circuit arrangement for converting a DC voltage present at DC voltage terminals into an alternating current which is supplied via AC voltage terminals. The circuit arrangement has a first series circuit which is connected to the DC voltage terminal and comprises a first electronic switch and an inductor. The circuit arrangement further comprises a plurality of second electronic switches. One of the DC voltage terminals and one of the AC voltage terminals are connected to a neutral conductor. The inductor is in the form of a twin inductor having two windings which are arranged tightly coupled to one another, wherein the first winding is connected in series with the first electronic switch and buffer-stores the energy produced via the electronic switch. The two windings are each connected to the AC voltage terminal which is not connected to the neutral conductor via one of the second electronic switches. One of these second electronic switches is closed during the half-waves of the alternating current with positive voltage between the AC voltage terminals, whereas the other of the second electronic switches is closed during the half-waves of the alternating current at negative voltage at the AC voltage terminals. Thus, the first winding and the second winding of the inductor alternatingly provide the alternating current formed by operating the first switch at a high frequency half-wave by half-wave. In one embodiment of the known circuit arrangement two series circuits each comprising a first electronic switch and an inductor are provided and connected in parallel to the two second electronic switches. Here, the first switches of the two series circuits are operated at an offset to have a more uniform flow of energy.
WO 2011/016449 A1 discloses a DC/DC converter comprising two groups of four switching elements. The four switching elements of one of the two groups are arranged in two half-bridges between the input lines of the DC/DC converter, and the four switching elements of the other of the two groups are arranged in two half-bridges between the output lines of the DC/DC converter. One center point of each half-bridge of the one group is connected to one center point of a half-bridge of the other group via a primary winding of a magnetic field cancellation type transformer, whereas the center point of the other half-bridge of the one group is connected to the center point of the other half-bridge of the other group via a secondary winding of the transformer. Further, one of the input lines is directly connected to one of the output lines. The DC/DC converter is capably of varying boosting and bucking ratios and a bidirectional boosting-bucking operation.
There still is a need of an inverter for converting a direct current flowing between two input lines into an alternating current flowing between two output lines which does not require a high performance LC-filter for removing a current ripple.